Ans: A
Since the range is $9$, then we have $3 \le m, n \le 9$.
Since the range is $9$, then we have $3 \le m, n \le 9$.
I must be true. For any values of $m$ and $n$ between $3$ and $9$ inclusive, the median is $8$.
II may not be true. Take $m = n = 3$. The mean
$\begin{array}{cl}
= & \dfrac{3\times 4 +8\times 3 + 10 + 12}{9} \\
= & \dfrac{58}{9} \\
\neq & 8
\end{array}$
III may not be true. Take $m = n = 3$, the mode is $3$ not $8$.
